Essential Cell Biology

Bruce Alberts, Dennis Bray, Karen Hopkin

Chapter 11

Membrane Structure - all with Video Answers

Educators

Chapter Questions

Problem 1

Water molecules are said "to reorganize into a cagelike structure" around hydrophobic compounds (e.g., see Figure $11-9$ ). This seems paradoxical because water molecules do not interact with the hydrophobic compound. So how could they "know" about its presence and change their behavior to interact differently with one another? Discuss this argument and, in doing so, develop a clear concept of what is meant by a "cagelike" structure. How does it compare to ice? Why would this cagelike structure be energetically unfavorable?

Kenneth Jordan

Kenneth Jordan

Numerade Educator

Problem 2

Five students in your class always sit together in the front row. This could be because (A) they really like each other or (B) nobody else in your class wants to sit next to them. Which explanation holds for the assembly of a lipid bilayer? Explain. Suppose, instead, that the other explanation held for lipid molecules. How would the properties of the lipid bilayer be different?

Michelle Bandeira

Michelle Bandeira

Numerade Educator

Problem 3

It seems paradoxical that a lipid bilayer can be fluid yet asymmetrical. Explain.

Aditya Sood

Numerade Educator

Problem 4

Explain why the polypeptide chain of most transmembrane proteins crosses the lipid bilayer as an $\alpha$ helix or a $\beta$ barrel.

Hailey Tomashek

Hailey Tomashek

Numerade Educator

Problem 5

For the two detergents shown in Figure $11-25,$ explain why the red portions of the molecules are hydrophilic and the blue portions hydrophobic. Draw a short stretch of a polypeptide chain made up of three amino acids with hydrophobic side chains (see Panel $2-5$ pp. $74-75$ ) and apply a similar color scheme.

Sana Riaz

Numerade Educator

Problem 6

Look carefully at the transmembrane proteins shown in Figure $11-29$ What can you say about their mobility in the membrane?

Aditya Sood

Numerade Educator

Problem 7

Describe the different methods that cells use to restrict proteins to specific regions of the plasma membrane. Is a membrane with many of its proteins restricted still fluid?

Aditya Sood

Numerade Educator

Problem 8

Which of the following statements are correct? Explain your
answers.
A. Lipids in a lipid bilayer spin rapidly around their long axis.
B. Lipids in a lipid bilayer rapidly exchange positions with one another in their own monolayer.
C. Lipids in a lipid bilayer do not flip-flop readily from one lipid monolayer to the other
D. Hydrogen bonds that form between lipid head groups and water molecules are continually broken and re-formed.
E. Glycolipids move between different membrane-enclosed compartments during their synthesis but remain restricted to one side of the lipid bilayer.
F. Margarine contains more saturated lipids than the vegetable oil from which it is made.
G. Some membrane proteins are enzymes.
H. The sugar layer that surrounds all cells makes cells more slippery.

Prashant Bana

Numerade Educator

Problem 9

What is meant by the term "two-dimensional fluid"?

Aditya Sood

Numerade Educator

Problem 10

The structure of a lipid bilayer is determined by the particular properties of its lipid molecules. What would happen if
A. Phospholipids had only one hydrocarbon tail instead of two?
B. The hydrocarbon tails were shorter than normal, say, about 10 carbon atoms long?
C. All of the hydrocarbon tails were saturated?
D. All of the hydrocarbon tails were unsaturated?
E. The bilayer contained a mixture of two kinds of phospholipid molecules, one with two saturated hydrocarbon tails and the other with two unsaturated hydrocarbon tails?
F. Each phospholipid molecule were covalently linked through the end carbon atom of one of its hydrocarbon tails to a phospholipid tail in the opposite monolayer?

Danielle Ashley

Danielle Ashley

Numerade Educator

Problem 11

What are the differences between a phospholipid molecule and a detergent molecule? How would the structure of a phospholipid molecule need to change to make it a detergent?

Aditya Sood

Numerade Educator

Problem 12

A. Membrane lipid molecules exchange places with their lipid neighbors every $10^{-7}$ second. A lipid molecule diffuses from one end of a 2 - $\mu$ m-long bacterial cell to the other in about 1 second. Are these two numbers in agreement (assume that the diameter of a lipid head group is about $0.5 \mathrm{nm}$ )? If not, can you think of a reason for the difference?
B. To get an appreciation for the great speed of molecular diffusion, assume that a lipid head group is about the size of a ping-pong ball $(4 \mathrm{cm}$ in diameter) and that the floor of your living room $(6 \mathrm{m} \times 6 \mathrm{m})$ is covered wall-to-wall with these balls. If two neighboring balls exchanged positions once every $10^{-7}$ second, what would their speed be in kilometers per hour? How long would it take for a ball to move from one side of the room to the opposite side?

Rabeya Zahid

Numerade Educator

Problem 13

Why does a red blood cell plasma membrane need transmembrane proteins?

Aditya Sood

Numerade Educator

Problem 14

Consider a transmembrane protein that forms a hydrophilic pore across the plasma membrane of a eukaryotic cell, allowing $\mathrm{Na}^{+}$ to enter the cell when it is activated upon binding a specific ligand on its extracellular side. It is made of five similar transmembrane subunits, each containing a membrane-spanning $\alpha$ helix with hydrophilic amino acid side chains on one surface of the helix and hydrophobic amino acid side chains on the opposite surface. Considering the function of the protein as a channel for $\mathrm{Na}^{+}$ ions to enter the cell, propose a possible arrangement of the five membrane-spanning $\alpha$ helices in the membrane.

Sana Riaz

Numerade Educator

Problem 15

In the membrane of a human red blood cell, the ratio of the mass of protein (average molecular weight 50,000 ) to phospholipid (molecular weight 800 ) to cholesterol (molecular weight 386 ) is about 2: 1: 1 . How many lipid molecules are there for every protein molecule?

Sana Riaz

Numerade Educator

Problem 16

Draw a schematic diagram that shows a close-up view of two plasma membranes as they come together during cell fusion, as shown in Figure $11-30 .$ Show membrane proteins in both cells that were labeled from the outside by the binding of differently colored fluorescent antibody molecules. Indicate in your drawing the fates of these color tags as the cells fuse. Will they remain on the outside of the hybrid cell after cell fusion and still be there after the mixing of membrane proteins that occurs during the incubation at $37^{\circ} \mathrm{C} ?$ How would the experimental outcome be different if the incubation were done at $0^{\circ} \mathrm{C} ?$

Sana Riaz

Numerade Educator

Problem 17

Compare the hydrophobic forces that hold a membrane protein in the lipid bilayer with those that help proteins fold into a unique three-dimensional structure.

Sana Riaz

Numerade Educator

Problem 18

Predict which one of the following organisms will have the highest percentage of unsaturated phospholipids in its membranes. Explain your answer.
A. Antarctic fish
B. Desert snake
C. Human being
D. Polar bear
E. Thermophilic bacterium that lives in hot springs at $100^{\circ} \mathrm{C}$

Michelle Bandeira

Michelle Bandeira

Numerade Educator

Problem 19

Which of the three 20 -amino-acid sequences listed below in the single-letter amino acid code is the most likely candidate to form a transmembrane region ( $\alpha$ helix) of a transmembrane protein? Explain your answer.
$\begin{array}{llllllllllllllll}\text { A. } & I & T & L & I & Y & F & G & N & M & S & S & V & T & Q & T & I & L & L & I & S\end{array}$
$\begin{array}{lllllllllll}\text { B. } & \text { L } & \text { L } & \text { L } & \text { I } & \text { F } & \text { F } & \text { G } & \text { V } & \text { M } & \text { A } & \text { L } & \text { V } & \text { I } & \text { V } & \text { V } & \text { I } & \text { L } & \text { L } & \text { m }\end{array}$
$\begin{array}{llllllllllll}C & L & L & K & K & F & F & R & D & M & A & A & V & H & E & T & I & L & E & E & S\end{array}$

Sana Riaz

Numerade Educator